Problem: totally unexpected significant imaginary frequencies are obtained (in a Frequencies run) where you are pretty convinced that all frequencies should be real.

Possible cause 1: problems with the electronic configuration. If there are competing configurations, the electronic states in the different displaced geometries may be different, resulting in energies and gradients belonging to different potential energy surfaces to be compared and combined into force constants (frequencies).

Check: orbital occupations and SCF convergence behavior: if the SCFs in the displaced geometries start with large errors and/or converge very slowly you are likely to have stumbled into different configurations, so that the results from the displaced geometries are incompatible.

Cure: This is a difficult situation that may require some experimenting and judicious manipulation of the various SCF options. The bottom line is that you should try anything you can to ensure that all involved geometries have the same electronic configuration. As long as you fail to achieve this, the results are meaningless.

Check: visualize the imaginary frequencies in ADFspectra and check that their respective normal modes correspond to movements that are expected to be have (nearly) flat energy profile.

Cure
:

restart geometry optimization with more strict convergence criteria. The default criterion on gradients 0.001 Hartree/Angstrom may be not strict enough for some systems. In such cases a value of 0.0001 is recommended, and for accuracy reasons use “good” numerical quality, and EXACTDENSITY (important for GGA’s).

Example input with strict settings using analytical frequencies, and a TZ2P basis set. Unlike the numerical frequencies, the analytical frequencies can be computed immediately after a geometry optimization by including both block keywords in the same input file.

Problem: successive displaced geometries in the logfile are numbered, but in your case these numbers make sudden jumps, like ‘0, 1, 2, 5, 6, 13...’

Cause: you’re using Cartesian displacements in a system that has some symmetry in its equilibrium geometry. The program skips the displacements of symmetry-equivalent atomic coordinates to save time. The displacement counts in the logfile do not run over the actually performed displacements but over all atomic coordinates that could be displaced if no use were made of symmetry properties.